This invention is directed at a method of manufacturing a non-volatile memory or storage, and more particularly to an improved method of manufacturing an electrically alterable read-only memory (EAROM) comprising a floating gate type field effect transistor wherein a part of the region of the floating gate and a part of the drain region overlap.
A memory of the kind just described has been generally known, and one example thereof is shown as a plan view in FIG. 1A and as a cross-sectional view in FIG. 1B taken along the line B--B of FIG. 1A. In these figures, a p-type silicon (Si) substrate 1 is illustrated along with a source region 2, enclosed by a dotted line, which is doped n.sup.+ -type and connected to a grounding terminal not shown. A drain region 3, also enclosed by a dotted line, is similarly doped n.sup.+ -type and connected to a bit line 4 made of aluminum (Al) wiring. A control gate 5 of polycrystalline silicon constitutes a word line 6. A floating gate 7 of polycrystalline silicon has a region which overlaps the control gate 5 and a region that overlaps the drain region 3 as illustrated in FIG. 1C which is a cross-sectional view taken along the line C--C of FIG. 1A. In these figures, the reference numerals 8, 9 and 10 denote insulation layers respectively.
In writing information into a non-volatile storage as depicted, electric voltage is applied to the word line 6 and bit line 4 respectively, and by utilizing tunnel effect, electrons in the drain region 3 are caused to be injected into the floating gate 7 where the electrons are accumulated. When reading the information, electric voltage is applied to the word line 6 and the bit line 4 to address, and current flows to the bit line 4 if electrons are accumulated in the floating gate 7 and the information has been written therein, and thus the information may be read. In order to erase the information, that is for the purpose of driving out the electrons accumulated in the floating gate 7, an electric voltage that is the reverse to the voltage applied for to write the information is applied to the word line 6 and bit line 4, and taking advantage of the tunnel effect, electrons are driven to that part of the drain region 3 overlapping the floating gate 7.
In a non-volatile memory device which functions as described above, the characteristics of an insulation layer where the floating gate 7 and the drain region 3 overlap greatly influence important characteristics of the memory such as retention and loss of the stored data. Electric charges pass through the insulation layer to enable writing and reading of data. If electric charges are trapped in the insulation layer and remain there, the insulation layer remains at a certain level of electric potential. Depending on the sign of the potential, the stored data is either difficult to erase or easily lost. This may place limitations on the frequency or number of times information may be written.